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ABSTRACT: Sequence Stratigraphy of the Upper Cretaceous (Cenomanian and Turonian) Tuscaloosa Group in the Subsurface of the Eastern Gulf Coastal Plain Area

Enrest A. Mancini, Berry H. Tew, Robert M. Mink

In the subsurface of the eastern Gulf Coastal plain area, the Tuscaloosa Group consists of approximately 1000 ft of nonmarine, marginal marine, and marine terrigenous clastic sediments. Based on regional stratigraphical, sedimentological, petrophysical, and paleontological data, three unconformity bounded depositional sequences reflecting relative changes in sea level and coastal onlap during the Late Cretaceous (Cenomanian and Turonian) are recognized for these strata. The depositional sequences probably correspond to the UZA-2.3, UZA-2.4, and UZA-2.5 global sequences. The lowermost depositional sequence is underlain by undifferentiated Lower Cretaceous terrigenous clastic deposits and consists of a basal type 2 unconformity, shelf-margin deposits (marginal marine sandst nes), transgressive and condensed section deposits (marine shelf to strandplain sandstones and claystones), and progradational regressive highstand deposits (strandplain to lagoonal sandstones and claystones). This sequence indicates that relative sea level rose in the middle Cenomanian and then fell during the late Cenomanian. The uppermost sequence consists of a basal type 2 unconformity, transgressive deposits (marine shelf sandstones), condensed section deposits (marine shelf claystones), and progradational regressive highstand deposits (marginal marine sandstones and claystones). This sequence reflects major sea level rise beginning in the late Cenomanian and continuing into the Turonian, followed by sea level fall in the Turonian. Within these depositional sequences, petroleum rese voirs occur principally in lowstand and transgressive deposits, but can occur in progradational regressive highstand deposits. Condensed section claystones are rich in herbaceous and algal organic matter and have the potential to be petroleum source rocks if buried under conditions favorable for hydrocarbon generation.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990